1. Field of the Invention
The present invention relates to a clamp for the bead core of a tire, such as a band, thread, bead core gripping means, or the like. The present invention also concerns a tire incorporating such a clamp in its structure and is directed to a method for making same.
2. Description of the Related Art
As is known, a tire includes the carcass, the tread band, and a belt structure between the tread band and the carcass. The carcass, usually at least a single ply, is turned out at its ends around a pair of bead cores. The bead cores, the ends of the carcass, and whatever filler that may be added between the bead cores and carcass work together as a unit to form the beads on either side of the tire.
When in use, a tire is placed on a wheel rim, which has two seats axially displaced from one another. The two beads on either side of the tire are designed to rest on the two rim seats. Each of the rim seats terminates in an end flange, which has an outermost diameter greater than the diameter of the wheel rim and also greater than the diameter of tire beads. The end flanges prevent the beads of the tire from slipping off of the rim during use.
Since each bead core and associated bead have smaller diameters than the outermost diameter of the vertical end flanges, the tire bead must be elliptically deformed so that the tire can be installed on the rim. Methods for elliptically deforming the tire bead are well known to those of ordinary skill in the art. Once the tire bead is sufficiently deformed that the elliptical axis of the bead is greater than the diameter of the vertical flange, the bead can be slipped over the end of the flange and the tire can be positioned on the rim.
Once on the rim, the tire beads are positioned one on each seat. The rim seats have gradually divergent, axial cross-sections. As a result, the beads are able to grip firmly onto their respective rim seats due to an elastic reaction between the bead cores and the gradually divergent rim seats as the bead cores are gradually forced onto the rim seats until they are finally positioned adjacent to the end flanges. The elastic gripping force of the beads is augmented once the tire is inflated and the external sides of the beads are pushed against the lateral flanges by the air pressure within the tire. The tire""s ability to grip firmly onto the tire rim is a fundamental safety feature of a tire""s design.
The bead core in a tire may take many different forms. For example, the bead core may comprise a plurality of plain metal wires (i.e., wires with a circular cross-section) bundled together. Alternatively, the bead core be made from a number of reinforcing elements that are strap-shaped or, in the case of large-sized tires, the bead core may include a number of quadrangularly or hexagonal sectioned metal straps. For large-sized tires, the bead core is formed by winding a metal strap repeatedly around a drum until several concentric coils are formed. When constructed in this manner, the bead core essentially is made of a first ring superposed by several others, each of which is formed by winding an additional layer of the metal strap onto the one that precedes it.
In another bead construction contemplated for large-sized tires, the bead comprises a group of radially superposed coils formed by winding a hexagonally-sectioned metal strap over itself to form radially superposed coils. The superposed coils form groups of rings arranged axially side by side. So that the adjacent rings may be wedged into one another, the radially superposed coils are radially grooved between one another. Lastly, the free, internal radial edges of the flanked coils lie on a straight line inclined, for example at 15xc2x0 for trucks and at 5xc2x0 for cars, to the wheel axis, allowing the beads, of which the coils are an integral part, to be mounted onto the respective bead seats, which have the same incline angle.
During the manufacture of a tire and during its normal operation, the bead cores of a tire are subjected to a variety of external stresses, such as high temperature and pressure. Mechanical forces act on the bead core as the individual components of a tire are assembled to form the final product. As a result, the different components of the bead core (i.e., the concentrically wound wires, straps, or tapes) may become misaligned or can lose the ability to resist torsional stresses. Should this occur, the bead core will not function adequately because the bead core will not be able to generate the necessary gripping force between itself and the rim seats, especially if the bead does not include suitable annular gripping means.
To compensate for this, clamps or gripping means are added around portions of the bead core. These clamps or gripping means function annularly to mutually compress the various elements of the bead core. They provide a reciprocal compacting force that helps the bead to firmly grip the rim seat. Many suitable clamps or gripping means for bead cores are known such as wire fabrics, nylon threads, high viscosity compounds, semivulcanized compounds applied around the bead core (for vehicle tires), or steel bands or metal strip coated with brass or zinc also disposed around the bead cores (for large-sized tires).
The clamp made from metal strip is first cut to a predetermined length, the two ends are then shaped in the form of hooks, the strip is wound onto itself around the bead, and the hooks are linked together. Alternatively, the ends of the strip may be soldered together. Finally, irrespective of how the ends of the clamp are connected to one another, the clamp may be pounded so that it is adequately banded over the bead.
Regardless of the manner in which the particular clamp or gripping means is formed, once assembled, the clamp forms an annular band corresponding to the peripheral dimension of the bead core. The band, just as with other similar clamps or gripping means, will exert a compacting force on the various elements of the bead core so that they do not become misaligned or loosened either during the manufacture of the tire or during operation of the tire.
While attempting to solve the problems with the prior art, it was observed that the clamps or similar elements of the bead core lose some of their gripping power over time. The reasons for this are unclear, but it is believed that stresses resulting from the handling of the bands during manufacture might play a significant role. Moreover, it is believed that loss of gripping strength may be attributable to the heating operation during vulcanization or during operation of the tire once installed on the tire rim. It is difficult to attribute the loss of gripping power of the band to any particular source because it does not correlate directly with any thermal phenomenon or even any mechanical phenomenon. Despite this, it was believed that a material for clamps could be found that was sensitive to the effects of temperature that also could compensate for the aforementioned loss of gripping strength.
Subsequently, it was thought that a material sensitive to temperature variations might be used in the manufacture of the bands or gripping means themselves so that the bands might react by generating a stress of contraction proportional to the variation in the gripping strength. In this way, the gripping strength of the clamp would be proportional to the need for a compacting force on the bead core elements. In other words, it was believed to be beneficial to use a material capable of providing a high gripping strength during the vulcanization process and during the operation of the tire so that the elements of the bead core would not be upset at the high temperatures and pressures experienced in each environment. Such a solution would also be consistent with the demand for a bead that tightly grips the rim seat at ambient temperatures but also would be consistent with a material that displays a certain degree of deformability at ambient temperature when it is necessary to deform the bead core so that it can be mounted to the tire rim.
It was then thought that the problem could be solved by resorting to annular clamps or gripping means made from an alloy of a shape-memory material. As a result, one embodiment of the present invention contemplates the inclusion of an annular clamp or gripping means made from a shape-memory alloy arranged around the bead core of a tire. Such an alloy for the gripping means is not only deformable at ambient temperature but contracts at a temperature above ambient to recover its previously memorized shape. Once the material contracts, it maintains this magnitude of the force of contraction as the material cools from the higher temperature to the ambient temperature.
Preferably, the gripping means comprises at least one wire. More particularly, the gripping means comprise at least one metal strap bent at the ends in the form of hooks which are linked together in the closed position around the bead core.
The present invention also provides a procedure for the manufacture of the clamp or gripping means around the bead core of a tire bead. The procedure includes a number of steps from winding the strip around the bead core, locking the ends of the strip together around the bead core to form a closed clamp or gripping means, and applying compression forces to the clamp to band the gripping means over the bead core. This process requires that the metal strip be made from a shape-memory alloy deformed by elongation relative to a previously memorized plane shape. The clamp is then heated to a predetermined temperature at which the shape-memory material transitions from a first structure to a second structure, different from the first, at which time the shape-memory alloy tends to recover the previously memorized shape and can exert a stress of contraction on the bead core.
Preferably, the procedure is characterized by heating the gripping means to a predetermined temperature As, which corresponds to the start of the transformation of the shape-memory material from a martensitic structure to an austenitic structure. Again preferably, the procedure is characterized by the fact that the temperature As is between 50 and 150xc2x0 C.
The procedure for manufacturing the bead core gripping means should be particularly suited to the vulcanization phase of tire manufacture. The vulcanization of a tire usually occurs at a temperature between 140 and 180xc2x0 C., which corresponds to a stress of contraction of the gripping means over the bead core of at least 200 MPa. As mentioned, this interaction is desired because it helps to keep the various bead core elements compact, so that such elements are not upset by their position.
Yet another aspect of the invention lies in manufacture of a tire having a carcass, a tread band, a belt structure between the tread band and carcass, a pair of bead cores, and at least one clamp or bead core gripping means around each bead core. The carcass includes a backing with ends unrolled surrounding the bead cores to form a pair of beads. The clamp or gripping means wrapped around the bead core is to be made of a shape-memory alloy. In this embodiment of the invention, it is contemplated that the bead core is formed of several rings or metal straps, radially staggered side by side, of radially superposed coils with a substantially hexagonal cross section.
Even more preferably, the tire of this particular embodiment of the present invention is characterized by the fact that, at temperature Af, which is between 70 and 200xc2x0 C., corresponding to the structure""s complete transformation from its martensitic state to its austenitic state, the bead core gripping means manifests a stress of contraction over the bead core of value "sgr"max, which is between 100 and 600 MPa.
Finally, as the beads heat up during normal operation on a tire rim, the magnitude of the above-mentioned stress favorably allows the various elements of the bead core to become more compacted, thus assuring that the tire beads grip onto the seats of the wheel rim.